The invention relates to ink-receptive coating compositions, particularly ink receptive coatings for inkjet papers, and recording medium prepared from such coatings, e.g., inkjet paper.
Inkjet printing comprises applying ink droplets to a recording medium, 1:1 usually paper. The most frequently used inkjet processes utilize thermal (bubble) and/or piezo electric processes which create and project a droplet of ink from a nozzle.
Inks used in inkjet printing processes are generally aqueous solutions containing a variety of functional additives. In general, these solutions contain azo dyes having a water solubility which can be specifically adjusted by the incorporation of hydrophilic side chains. These dyes are generally anionic.
These processes and ink formulations have to be taken into account when preparing the recording medium. Indeed, one typically strives to address the following criteria when designing a recording medium for inkjet processes.
1. Controlled ink absorption of the ink droplets so that the absorbed ink forms circular spots of ink with smooth peripheral lines thereby imparting high resolution.
2. High absorption rate of ink to reduce smearing.
3. Low penetration depth of the dyes so as to form high optical density images and reduce xe2x80x9cshow-throughxe2x80x9d or penetration of the ink spots.
4. High affinity between the color dye and print medium to enhance color fastness.
5. Color appearance and fidelity for a spectrum of dye colors.
6. High whiteness to achieve good contrast to the ink.
7. Resistance to yellowing and fading of colors.
8. Resistance to aqueous or alcohol solutions.
9. High ink absorbing capacity so as to receive the amounts of ink necessary for dense images.
10. Minimal dusting from the final recording medium product.
The structure and composition of the recording medium significantly contributes towards meeting these criteria. Typically, recording media such as paper which has only been sized on the surface cannot meet these needs. As a result, specially designed inkjet printing papers have been developed. These papers consist of a paper substrate on which an ink receptive coating layer has been applied to at least one surface. These coatings can be applied while the paper is being made, i.e., xe2x80x9conlinexe2x80x9d. The coatings can also be applied after the paper has been made. That process is sometimes referred to as xe2x80x9cofflinexe2x80x9d.
A number of binders have been used to coat these papers and have been identified as having an effect on one or more of the above properties. The binders generally include water soluble film formers and/or latex polymers. WO 97/01447 discloses using water soluble polyvinyl alcohol and various types of latex such as vinylacetate latex, including vinyl acetate copolymers and homopolymers. The binders can be cationic, anionic and/or nonionic. Polyvinyl alcohol is a typical binder for pigmented coatings. Certain polyvinyl alcohols are not only good film formers which bind the pigment particles, but they also are hydrophilic and help limit diffusion of the inks within the pigment coating. See EP 825 031.
The ink receptive coating also can contain highly absorptive pigments in order to assist meeting the earlier mentioned criteria. Highly porous silica pigments have been especially successful in meeting those needs. However, when porous silicas are combined with conventional binders, e.g., polyvinyl alcohol, there is a maximum amount of silica that can be added before the formulation becomes too viscous for most practical uses. Indeed, typical binders, such as polyvinyl alcohol, are only available at low solids content, e.g., 10% by weight solids. As a result, most silica-containing formulations are prepared at a low solids content after dispersion. Such low solids formulations are not the most efficient or economical to use in current paper making processes.
For example, paper is made by processing a continuously moving web of substrate. As a result, an efficient application of a coating to this web comprises applying the coating directly xe2x80x9conlinexe2x80x9d during the manufacturing process, or applying the coating offline after the paper has been made by w applying the coating to the paper as it is run off a spool. With either method, it has been established that a certain amount of solids needs to be applied to the web or paper to create coating integrity, as well as to impart the print properties mentioned above. Moreover, the solids content and the viscosity of the coating determine how fast the coating can be efficiently applied to the moving web and still result in an adequate coating. In general, the higher the solids and the lower the viscosity, the faster the coating can be applied to the moving web. Reasonable line speeds for online processes are 200 feet per minute or faster. The faster the whole process can be run, the more efficient the whole manufacturing process becomes. On the other hand, a low solids formulation usually means a slower line speed and, as mentioned above, attempts to add more solids to conventional low solids silica-containing formulations has resulted in formulations which are too viscous to be applied online.
Certain low solids coating composition also tend to rub off and create dust after the coating is dried, and can result in a coating that does not bind to the substrate. Conventional low solids formulations also typically need additional components such as dye mordants due to the anionic nature of inkjet dyes and/or binders for some formulations. These coatings also incur increased expenses from drying lower solids materials.
It also is difficult to reduce or minimize the disadvantages of low solids coatings while at the same time maintaining or maximizing the ink receptive properties desired, especially when the coating also contains additional components, e.g., cationic polymeric dye mordants such as polydiallyl dimethyl ammonium chloride. See U.S. Pat. No. 5,494,759 and EP 799 136. These additives impart water fastness to the dyes that are applied to the coating. Indeed, it has been generally acknowledged that it is difficult to achieve coating formulations having more than 20% solids by weight. See U.S. Pat. No. 5,827,363. Accordingly, there have been attempts to solve the problems associated with those formulations.
For example, DE 195 34 327 discloses preparing coatings containing cationic and/or nonionogenic binders in combination with pigment particles having cationic surface charge to increase solids content. This patent suggests that formulations having solids content up to 70% by weight can be obtained using these formulas. However, the pigments disclosed in DE 195 34 327 include calcium carbonate and fumed silicas. These pigments have limited porosity, and it is envisioned that the coating""s overall ink capacity is limited when using these pigments. It also is envisioned that higher than necessary pigment will be needed to obtain the derived ink absorption properties. In addition, media prepared from certain cationic binders such as those suggested in this patent also could benefit from some improvements in terms of print performance.
U.S. Pat. No. 5,720,806 discloses coating compositions comprising 1-30% solids by weight. This patent, however, fails to disclose specific examples of binder formulations, much less disclose a formula which achieves a high level of solids in a formulation and yet has low viscosity and results in suitable ink receptive coatings. U.S. Pat. No. 5,720,806 is concerned with improving coating formulations by adding small amounts of alumina to silica pigment, i.e., less than 1% by weight based on the silica.
Therefore, there is still a need for a coating having a relatively higher solids content, e.g., greater than 20%, so that the coating is not only less viscous to meet the needs of manufacturing the medium in a more efficient manner, but also imparts properties that meet the print performance needs for recording medium in the inkjet area.
It is an object of this invention to address several of the aforementioned problems associated with ink-receptive coatings comprising porous pigments such as silica-based pigments. These problems include low formulation solids content, lack of adherence of the dried coating to the substrate paper (coating dusting) and poor inkjet printability. These problems are especially noted for coating formulations applied online as paper is being made.
It has been discovered that by combining (a) a nonionic latex with (b) conventional water-soluble polymer binders and (c) cationic porous inorganic oxides, one can obtain a coating composition having a solids content of at least 20% by weight and a Brookfield viscosity of 5000 centipose or less. Indeed, one can obtain a solids content of about 25 to about 40% by weight, and preferably 30% to 40% solids, and Brookfield viscosities less than 2000 centipose using this composition. It has also been shown that the print properties required of inkjet paper can be met using these formulations. Cationic dye mordants can also be employed without significantly affecting the desired solids content and coating viscosity.